Only one diode will conduct per phase cycle, the diode with the most voltage takes first phase, then as its phase decays, and the other phase increases, the second diode takes over, then onto the third phase , then back to the first, the same on the negative cycles.

in a full bridge, only two are ever in full conduction( pos and neg cyles) otherwise you would be shorting out the phases.

If you view the voltage waveform for the three phases on one graph (such as here) you will see at which point each phase has a higher voltage and polarity as compared to the other phase the diode is connected to. That will tell you which diode is conducting.